Improve srcsets and add where missing

[cathysarisky]

Hello! Offering the following improvements to Solo image handling:

1. Logo: Previously no srcset, loaded full size logo (which could be /very/ big. The challenge is that we don't have the dimensions for the user's logo, and the height restriction (48px) is driving how big it can be. It's mostly loading the 720w version, because I could imaging a case with a very wide and short logo where that'd be appropriate. An improvement over loading a worst-case 2000px+ image, at least.
2. Hero image: The code previously loaded a very large image, in case the "Large background" theme option was selected. I split out the logic, so that a smaller image loads for the more commonly-used side by side layout.
3. Loop images: Similar to the hero image, it was loading a very large image in case the Parallax view was used, but the classic layout needed a much smaller image. I split the logic.